This invention relates to method and apparatus for effecting control of an expander used to drive a generator for generating electrical power. More specifically this invention concerns controlling the flow of the driving fluid to the expander to effectively regulate the rotational velocity of the generator and consequently the frequency of the electrical power generated.
In many applications hot gas is supplied to a power recovery expander wherein the gas is used to drive the expander generating shaft work. This shaft work is then typically converted to some other type power. In the herein instance this shaft work is utilized to drive a rotary electrical power generator.
Gas expanders may be found in association with the fluid catalytic cracking processes employed in the petroleum industry and are used to recover energy from waste heat gas streams. Other applications include coal gasification and other industrial processes wherein hot gas is generated.
The utilization of hot high energy gas for driving an expander to effectively remove some of the energy contained in the gas is not new. Previous systems have utilized a large fast acting valve to control the flow of gas to the expander to regulate operation of the expander. A valve of this size must be very fast acting to appropriately control the amount of hot gas being supplied to the expander to obtain the desired rotational velocity and to regulate the generator to obtain synchronous electrical power generation. The generator is typically connected to an electrical power grid having other power sources. In a typical electrical power grid alternating current is supplied at a desired frequency such as 60 cycles per second. Hence the expander must be powered to operate at a rotational speed in combination with the generator such that electrical power will also be supplied to the grid from the generator at 60 cycles per second and such that said electrical power will be provided synchronously with the power on the grid.
Controlling the flow of hot gas to the expander utilizing a fast acting control valve has not been fully satisfactory as concerns reliability. A typical valve might be a 52 inch or larger valve and it might require a full stroke of the valve gate between the fully opened and closed positions to occur within less than half a second. A valve of this size having this fast stroke of the gate mechanism is prone to failure and prone to control problems due to the accuracy necessary.
The herein invention concerns the utilization of parallel hot gas conduits to the expander. A first much larger conduit is controlled with a large relatively slow acting valve. A second smaller conduit has a smaller fast acting valve such that the combination of the two valves allows the desired control to be achieved. Additionally by utilizing a relatively slow acting valve with a relatively fast acting valve when one valve tends to overcompensate the other valve tends to undercompensate yielding damped second order response characteristics and allowing for effective overall control of the system.
Additionally a control system may be provided such that the smaller fast acting valve is connected to a reverse active frequency control and such that the larger slow acting valve is connected to a forward acting frequency control such that each valve acts with an opposite bias to create the desired damped response.